Crystal structures and metal uptake capacity of 10aa-manganates: An overview

Marine and synthetic 10aa-manganates and terrestrial todorokites have similar tunnel structures. Each tunnel wall consists of 3 edge-shared [(Mn4+, Me(2+,3+), Mn2+)(O2-, OH-)6] octahedral strings along the tunnel, where Me(2+,3+) represents transition cations such as Fe3+, Co2+, Ni2+ and Cu2+. A ceiling or floor is composed of n edge-shared [Mn4+O2-6] octahedral strings along the tunnel. A tunnel is filled mainly by n-2 edge-shared [Me(1+,2+)(OH-, H2O)6] octahedral strings, where Me(1+,2+) stands for alkali and alkaline earth cations such as Na+, Ca2+ and Ba2+. The n numbers are invariant along the direction normal to the ceilings except where lattice defects occur, but vary randomly along the direction normal to the walls. The interlayer strength of the phases increases with the substitution of Me(2+,3+) for Mn2+ ions, the oxidation of Mn2+ to Mn4+ ions and the concomitant change of their OH- to O2- ligands in the walls. The ideal lattice of these phases appears to be orthorhombic, but some crystals may be monoclinic mainly because of inhomogeneous cation distributions in the walls and wall distortion.The crystallochemical formulae for the phases can be generalized asMe(1+,2+)